This invention relates generally to refining of steel. More particularly, this invention relates to processes for refinement of silicon-bearing Al-killed and Al—Si dual killed steel to be directly cast in a continuous slab caster.
In continuous slab casting, the continuous caster is comprised of a tundish and an oscillating mold, in addition to a shroud and submerged entry nozzle. The molten steel in the ladle is poured into a tundish and then poured vertically through the submerged entry nozzle into a hollow water-cooled oscillating mold, and continuously cast slabs are withdrawn horizontally from the bottom of the mold. Refractory shrouds are used to transfer the molten steel from the ladle to the tundish, and then to the submerged entry nozzle and the mold, to avoid oxidation of the molten steel through contact with air. The shroud between the tundish and the mold feeds through the submerged entry nozzle, and is regulated by a stopper rod.
The continuous slab caster produces wide rectangular strands of large cross-section, which are cut off into slabs to be hot rolled and cold rolled for use as material for sheet and plate. Thick slabs for flat-rolled products usually have an as-cast thickness of 100 to 250 mm. Thin slabs for flat-rolled products usually have an as-cast thickness of 30 to 100 mm. The slab caster is usually used in conjunction with an electric arc furnace or basic oxygen furnace, where the hot metal in produced for the caster.
Steel for continuous casting may be subjected to deoxidation treatment usually in a ladle prior to casting. Deoxidizing the molten steel in a ladle metallurgy furnace (LMF) or Vacuum Tank Degassed (VTD) to a desired oxygen level is typical. Aluminum (or a combination of Al and Si) has been widely used as a deoxidizer and grain size controller in the manufacture of steels. Aluminum acts as a sacrificial metal which combines with oxygen to form a stable aluminum oxide, which migrates into the slag. Aluminum is a particularly desirable material for this purpose because it can be safely stored, handled and transported at ambient temperature, and, it is reactive as an oxidizing agent with steel at steelmaking temperatures.
Most thin slab casting and plating grades of steel are typically Al-killed steels. In some cases a combination of Al and Si is used to kill the steel. While this steel can be cast “as is” in large slab casters, further treatment is required in thin slab casters to avoid clogging or choking of submerged entry nozzles. One established practice in thin slab casting is to modify alumina and spinel inclusions by treatment with calcium to provide for more liquidity. With proper calcium treatment, the majority of the solid alumina (Al2O3) and/or spinel (MgAl2O4) inclusions are modified to liquid inclusions and casting is performed with acceptable surface quality to the cast slab. For continuous casting in a thin slab caster, 600 feet (182.9 m) of calcium wire has been found sufficient for a 170 ton (154 tons metric) ladle to add the calcium to avoid nozzle clogging (about 0.134 lb/ton, 0.067 kg/ton metric). 600 feet (182.9 m) of calcium wire contains about 22.5 lbs (10.2 kg) of calcium and is equivalent to about 16.8 ppm effective calcium in the refined steel. The recovery of calcium in the steel from calcium wire is less than 100% so that the effective calcium will be less than the amount added.
There are two main grades of silicon-bearing steels for sheets and plate steels made in a thin slab caster:                Silicon-restricted steel typically with less than 0.03% silicon Generally ferrosilicon or silicomanganese is not added        Silicon-bearing steel typically with about 0.1% to 1.5% silicon Silicomanganese and/or ferrosilicon is added to achieve the desired silicon content.        
Problems with stopper rod wear associated with excessive Ca-addition have been observed in silicon-bearing steels where ferrosilicon and/or Silicomanganese have been added to achieve the desired silicon concentration in the finished steel. In a “Study of Casting Issues using Rapid Inclusion Identification and Analysis”, Story, et al., AISTech 2006 Proceedings, Vol. 1, pp. 879-889, it was determined that ferrosilicon can contain calcium in addition to silicon and other alloying elements. To address stopper rod wear, Story et al. discussed using high purity ferrosilicon containing about 0.024% calcium.